Our objectives are to qualitatively identify and quantitatively define changing cell surface properties such as adhesiveness, motility, and cell contact behavior of pre-implantation mouse embryos as a function of their developmental age. The developmental period of most of our experimetal focus is the morula - to - blastocyst transformation (also termed blastulation or cavitation) which occurs between 60 to 90 hours post coitum. Presently our research includes: (1) development of an assay to measure surface adhesiveness of intact but denuded (removal of zona pellucida) mouse embryos versus their developmental age, (2) determination of dose-response effects of cytochalasin B on developing mouse embryos in vitro, (3) acquisition of quantitative ultrastructural data on the t-W32/t-W32 (also known as t-12/t-12) lethal mutant which undergoes developmental arrest at the morula stage especially as these data relate to cell surface/cell cortex phenomena, and (4) further characterization of the generation of the t-W32/t-W32 lethal syndrome especially in terms of the precise chronology of abnormal events and the clarification of these events as primary, secondary, or tertiary genetic defects.